DFB lasers are a variety of lasers which include one or more Bragg gratings which act as reflection elements within a laser active region. This technique of co-locating the gain medium and the feedback grating is applicable to fibre lasers such as those which employ a gain medium that has been doped with erbium.
An example of a prior art DFB fibre laser is illustrated in FIG. 1. Fibre laser 100 includes a doped fibre 110 and Bragg grating 120 incorporating a phase discontinuity located in the middle section 130 of grating 120. The Bragg grating is provided by a UV induced periodic spatial variation of the refractive index of the gain medium. Other techniques which provide a Bragg grating structure include periodic modulation of the gain or loss of the active region or potentially the cutting of a periodic pattern of grooves into the cladding of the fibre might also conceivably be used.
Fibre laser 100 is activated by optical pumping 140 which involves pumping light having a wavelength that matches with the appropriate absorption band of the active material or gain medium through a passive fibre connected to fibre laser 100. This arrangement of the Bragg grating 120 and gain medium provides optical feedback at approximately the Bragg wavelength λB characterised by the relation λB=2neffΛ where Λ is the period of the grating and neff is the effective refractive index of the fibre mode.
The grating is characterised by a complex coupling coefficient κ(z)=πΔn(z)e−iφ(z)/λ where Δn is the refractive index modulation and φ(z) is the phase error associated with the grating and where z is a measure of the longitudinal distance along the fibre. Accordingly the spectral width of the grating reflection is proportional to |κ|.
As illustrated figuratively in FIG. 2, a π phase shift is introduced into the middle section 130 of grating 120. The introduction of this phase shift ensures a lowest threshold, highly confined fundamental laser mode operating at essentially the Bragg wavelength λB. The typical field distribution of such a laser is shown in FIG. 3 where it can be seen that the field has a maximum at the location of the phase shift and decays exponentially away from the centre of grating 120. The spatial width of the field distribution depends on |κ| and defines the overall device length L which in practice is usually a few centimeters.
One of the major applications of a DFB fibre laser is to incorporate a number of fibre lasers into one continuous fibre to form a fibre laser array. Each of the fibre lasers are tuned to operate at slightly different wavelengths λB1, λB2 etc with the advantage that optical pumping at a single wavelength may be employed to cause each of the DFB fibre laser sections to lase. This provides a means for wave division multiplexing as laser emissions from each fibre laser section travel down the common fibre and may be sampled using interferometric processing downstream.
Arrays of DFB fibre lasers of this type have been employed in a number of applications including sensor arrays where the wavelength output of each fibre laser varies according to the local value of a physical characteristic of the environment such as the temperature or level of sound, to uses such as multi-wavelength laser sources. Clearly, the ability of each fibre laser section to emit light essentially at the respective Bragg wavelength is critical as each of the wavelengths λB1, λB2 etc. will be tightly spaced due to the finite emission band-width available to the gain medium, which must be similar for each laser due to the requirement that each fibre laser is activated by pump light having the same wavelength.
However, DFB fibre lasers have a number of disadvantages which directly affect the performance of fibre laser arrays based on a number of fibre laser sections. Although the Bragg grating is designed to reflect light in only a narrow band about the Bragg wavelength λB and to be essentially transparent outside the band there is in practice out of band reflection associated with the side-lobes of the Bragg grating.
The out of band reflection r(ν) is characterised by the relationship,
      r    ⁡          (      v      )        =      -                  ∫        0        L            ⁢                                    κ            ⁡                          (              z              )                                ·                      exp            ⁡                          (                                                -                  ⅈ                                ⁢                                                                  ⁢                2                ⁢                π                ⁢                                                                  ⁢                vz                            )                                      ⁢                                  ⁢                  ⅆ          z                    where ν is the detuning from the Bragg frequency and is defined by
  v  =      2    ⁢                  n        eff            ·              [                              1            λ                    -                      1                          λ              B                                      ]            for ν>|κ|. When two or more DFB fibre lasers are connected to the same fibre, this out of band reflection results in a fraction of light from a given fibre laser section being reflected by another fibre laser section thereby causing a shift Δλ from the Bragg wavelength λB for that particular fibre laser section.
For distance d between each fibre laser section this wavelength shift is approximated byΔλ=λB2κ|r|e−κL sin(2πd/λB−φr)/πwhere φr is the phase of the out of band reflection r(ν) (i.e. r(ν)=|r|eiφr) from the adjacent laser. Accordingly, the laser wavelength will be sensitive to both small changes in distance d between the fibre laser sections and the reflection coefficient r(ν) from the adjacent lasers. Clearly, this is undesirable in the example of a sensor array as the intent is to measure changes to the laser wavelength caused by local changes to the Bragg wavelength of the grating of the respective fibre laser section.
To address this issue of undesirable wavelength sensitivity, the physical length L of the grating structure can be increased. However, this has the obvious disadvantage of lengthening the fibre laser array where compactness is often a major requirement. In addition where the fibre laser sections are being employed in a sensor array such as an acoustic sensor, lengthening of each fibre laser section implies that a sample is taken from a distributed region as opposed to the fibre laser section acting as a point sensor. Often a sensor design will also require multiple point sensors in close proximity and lengthening the grating structure for each fibre laser section can greatly add to the mechanical constraints in dealing with such a sensor array.
It is an object of the invention to provide a DFB laser having improved characteristics that enable the incorporation of these devices into multiple DFB laser arrangements.